Clean up `topics` vs `practices` in `config.json`

Per the current docs, the track-level config.json's per-exercise topics key isn't used for anything. This has the advantage that the items there aren't constrained by cross-checks, but the disadvantage that we're not fully exposing to the website the information we have about each exercise.

The general procedure we want to follow goes like this:

• Collect the list of all current concept exercises. Currently:

  $ jq -r '.concepts[] | .slug' config.json | sort -u
  booleans
  box
  char
  collect
  destructuring
  entry-api
  enums
  floating-point-numbers
  fold
  functions
  hashmap
  integers
  loops
  match-basics
  mutability
  option
  references
  strings
  string-slices
  string-vs-str
  structs
  tuples
  vec-macro
  vec-stack
  

• For each exercise among the practice exercises:
  • For each item in topics
    • If the topic is a current concept, move it from topics to practices

This process needs to be repeated periodically as we add concepts, so PRs addressing this issue should not close it.